The present invention relates to a process for preparing a vacuum switch tube.
A vacuum switch tube is generally constructed in such a manner that a pair of electrodes is received in a vacuum envelope made of an insulating material so as to be connectable and separable with respect to each other by the movement of a bellows and the vacuum envelope is sealed under vacuum condition.
In the conventional process of preparing a vacuum switch tube, sealing of a vacuum switch tube has been conducted after exhausting air from an exhaust tube. Recently, there has been proposed such a method that brazing is carried out under a vacuum condition without using the exhaust tube. It is mainly because (1) the tip of the exhaust tube, being relatively weak, may be damaged during manufacturing steps of the vacuum switch tube, (2) presence of the exhaust tube prevents easy assembly when the vacuum switch tube is assembled into a current interrupter and (3) brazing operation under vacuum condition reduces the number of manufacturing steps because baking and sealing operations are simultaneously done.
As a process for preparing a vacuum switch tube without having an exhaust tube, it has been known to use a process in which a pre-assembled vacuum switch tube is put into a vacuum brazing furnace; the interior of the furnace is brought to a pressure of 10.sup.-4 Torr or less and a temperature of 400.degree. C.-600.degree. C. whereby air in the pre-assembled vacuum tube is evacuated through a clearance formed by the provision of a brazing material and then the brazing material is melted at a brazing temperature to accomplish the bonding operation by brazing; thus the vacuum switch tube is sealed.
In this case, there a rises a problem of the structure of an air exhausting part. Though an air exhausting passage is formed by a clearance formed near a brazing material put in a pre-assembled vacuum switch tube, if air-exhausting resistance is high at the clearance, the interior of the vacuum switch tube can not be brought to a high vacuum condition. Various attempts have been made to improve the exhausting of air from the vacuum switch tube. However, these attempts fail to sufficiently reduce the resistance of the air exhaust thereby being inacceptable in practical use.
Further, although there was an attempt to reduce air-exhausting resistance by providing a corrugated plate of brazing material to form a clearance in a portion to be brazed, the corrugated plate was apt to collapse and be deformed during assembling operation.